The present application relates to a winding electrode body in which a positive electrode and a negative electrode are wound together with separators, a nonaqueous electrolyte secondary battery using the winding electrode body, and a method for manufacturing a winding electrode body.
Heretofore, a large number of portable electronic apparatuses, such as a camera-integrated videotape recorder, a mobile phone, and a portable computer, have been introduced to the market, and the reduction in size and weight of the apparatuses has been pursued. Concomitant with this trend, as portable power sources of electronic apparatuses, batteries, in particular, secondary batteries have been actively developed. Among the secondary batteries, a lithium ion secondary battery has drawn attention as a battery capable of realizing a high energy density.
A lithium ion secondary battery includes an electrode body in which at least one positive electrode and at least one negative electrode are wound or laminated together with separators, a nonaqueous electrolytic solution, and an exterior package including a metal can made of aluminum or the like or an exterior member such as a laminate film, which accommodates the electrode body and the nonaqueous electrolytic solution.
Since the nonaqueous electrolytic solution contains a flammable nonaqueous solvent (such as a carbonate ester-based organic solvent), smoking and/or ignition may dangerously occur due to an external or an internal (such as an internal short circuit) factor; hence, one of important subjects of the secondary battery is to ensure the safety.
For example, in a lithium ion secondary battery, metal lithium used as an active material may be deformed due to charge-discharge cycles of the battery to generate dendrites, and in some cases, these dendrites may form a short circuit bridge to cause an internal short circuit.
In addition, for example, when a lithium ion secondary battery is heated in a heating test or the like, separators may shrink due to the heat, and in some cases, a positive electrode and a negative electrode may be directly brought into contact with each other to cause an internal short circuit. When the internal short circuit occurs, the thermal runaway may be induced as described above, and hence it is not preferable from a safety point of view.
As a technique to avoid an internal short circuit, for example, in order to prevent the generation of dendrites, a cylindrical-type lithium ion secondary battery has been disclosed in which a positive electrode and a negative electrode are wound together with belt-shaped separators each having a width larger than that of each sheet of the above electrodes, end portions of the separators protruding from the top and the bottom (or the right and the left) ends of the two wound electrode sheets in the axial direction thereof are folded in a winding axial direction, and the folded portion of each side is heat-sealed so that the positive electrode and the negative electrode are covered with the separators (see Japanese Unexamined Patent Application Publication No. 1-122574).
In addition, a laminate-type lithium ion secondary battery has also been disclosed which uses a laminate type electrode body in which positive electrodes and negative electrodes are laminated to each other with separators larger than of the two types of electrode sheets interposed therebetween, and in which the two types of electrode sheets are covered by adhering end portions of the separators protruding from each of two sides of the two types of electrodes (see Japanese Unexamined Patent Application Publication No. 2002-208442).